Axial flow gas turbine engines include a compression section, a combustion section and a turbine section. A flow path for working medium gases extends axially through these sections of the engine. The working medium gases are compressed in the compression section and burned with fuel in the combustion section to add energy to the gases. The hot, high pressure gases are expanded through the turbine section to produce useful work.
Rotor assemblies extend axially through the engine to transfer the energy required for compressing the working medium gases from the turbine section to the compression section. The rotor assembly in the turbine section includes one or more rotor disks. An array of rotor blades attached to the rotor disk extends outwardly from the disk across the working medium flow path. As the hot working medium gases are flowed through the turbine section of the machine, the gases exert a force on the rotor blades driving the array of rotor blades and the rotor disk at high speeds about the axis of rotation. The high speed rotation of each rotor disk and array of rotor blades imposes operating stresses on the rotor blades and the rotor disk. These stresses are especially severe in the disk near the root region of the rotor blade where the root engages a corresponding slot in the disk.
In addition to the stresses caused by the rotational forces, the airfoil section of each rotor blade is bathed in the hot working medium gas causing thermal stresses in the airfoils which affect the structural integrity and fatigue life of the airfoil. These thermal stresses have been a source of concern since the introduction of gas turbine engines because of the need to operate the engines at high temperatures to maximize engine efficiency. For example, the airfoils in the turbines of such engines may see temperatures in the working medium gases as high as twenty-five hundred degrees Fahrenheit (2500.degree. F.). The blades of these engines are typically cooled to preserve the structural integrity and the fatigue life of the airfoil by reducing the level of thermal stresses in the airfoil.
One early approach to airfoil cooling is shown in U.S. Pat. No. 2,648,520 issued to Schmitt entitled "Air Cooled Turbine Blade". Schmitt shows a coolable rotor blade. A shell defines the airfoil surface. The rotor blade has a core to support the shell and filler material to provide a heat transfer surface between the core and the shell. Cooling air is flowed through the blade to cool the blade and receives heat from the shell and from the filler material.
A later approach employed transpiration cooled turbine blades such as are shown in U.S. Pat. No. 3,067,982 entitled "Porous Wall Turbine Blades and Method of Manufacture" issued to Wheeler; U.S. Pat. No. 3,402,913 entitled "Method of Controlling The Permeability Of A Porous Material and Turbine Blade Formed Thereby" issued to Kump et al.; and U.S. Pat. No. 3,567,333 entitled "Gas Turbine Blade" issued to DeFeo. As shown in these patents, transpiration cooling is provided by flowing cooling air through a porous shell of the rotor blade. As in the early Schmitt patent, an inner core or strut is provided to support the porous shell.
U.S. Pat. No. 4,033,792 entitled "Composite Single Crystal Article" issued to Giamei discloses a coolable rotor blade having an inner core and airfoil elements which are supported by the core to form an aerodynamic flow directing surface. The core element has two passages for a cooling fluid. The inner core material is selected for high temperature strength. The airfoil material is selected for oxidation and corrosion resistance. The rotor blade is made by inserting the core and the airfoil elements into a recess in a base member and bonding together the core, the airfoil elements, and the base member.
As time passed more sophisticated approaches were developed for cooling the rotor blade. One of these approaches employed a one-piece casting having tortuous cooling air passages. Such a rotor blade is exemplified in the structure shown in U.S. Pat. No. 4,073,599 entitled "Hollow Turbine Blade Tip Closure" issued to Allen et al. Allen et al. discloses the use of intricate cooling passages coupled with other techniques to cool the airfoil. For example, the leading edge region in Allen is cooled by impingement cooling followed by the discharge of the cooling air through a spanwisely extending passage in the leading edge region of the blade.
The cooling of turbine airfoils using intricate cooling passages is the subject of many of the latest patents such as: U.S. Pat. No. 4,177,010 issued to Greaves et al. entitled "Cooling Rotor Blade For A Gas Turbine Engine"; U.S. Pat. No. 4,180,373 issued to Moore et al. entitled "Turbine Blade"; U.S. Pat. No. 4,224,011 issued to Dodd et al. entitled "Cooled Rotor Blade For A Gas Turbine Engine"; and U.S. Pat. No. 4,278,400 issued to Yamarik et al. Entitled "Coolable Rotor Blade". These rotor blades are typified by cooling air passages having dimensions which are many times greater than the thickness of the walls of the airfoil of the blade.
The orientation of the coolable airfoil of the rotor blade to the approaching working medium gases is constrained by aerodynamic considerations. These aerodynamic considerations require orienting the airfoil with respect to the approaching flow to remove the right amount of work from the approaching flow and to redirect the flow to the next array of airfoils in an efficient manner. The root of the rotor blade is oriented with respect to the airfoil to ensure that acceptable levels of stress are not exceeded in the critical transition region between the airfoil and the root of the rotor blade. Significant stress concentrations do occur in this region because of the transition from an elongated thin plate (airfoil) to the shorter, broader plate (root) which engages the rotor disk.
In modern gas turbine engines, the root is typically angled with respect to the axis of rotation to more closely align the root to the airfoil to reduce these stress concentrations. Because the orientation of the root is constrained by the orientation of the airfoil, the root is not optimally oriented with respect to the disk but rather is angled with respect to the face of the disk. This angles the slot in the disk with respect to the axis of rotation of the disk and results in the rotor blade exerting a twisting force which imposes a bending moment on the periphery of the disk. As a result, the stresses on one side of the slot in the disk are high at the leading edge of the disk and low at the trailing edge whereas on the opposite side of the slot the stresses are high at the trailing edge and low at the leading edge. A rotor blade disposed in a slot which is parallel to the axis of rotation does not exert such severe twisting forces on the disk and accordingly, the rotational load results in a uniform level of stress in the disk. It has long been recognized that slots in the disk which are substantially parallel to the axis of rotation result in a relatively uniform rotational stress in the disk thereby reducing the maximum level of stress in the rim of the disk and enable either a greater fatigue life in the disk or a disk of lighter weight. Nevertheless, the slots in the disk have typically been angled with respect to the axis of rotation because of the considerations of fatigue life in the rotor blade.
Any slot which extends across the disk destroys the hoop strength of the rim of the disk by interrupting the circumferential continuity of the rim. As a result, the disk material between slots cannot support itself against rotational forces. A circumferentially continuous hoop of material inwardly of the interrupted hoop is provided by the disk to support the material between the slots. The interrupted hoop of material between the slots is referred to as the "dead rim" of the disk. The weight of the rotor assembly beyond the continuous hoop is referred to as the "dead rim weight".
The disk is typically formed of high strength material such as nickel and cobalt base superalloys. These superalloys have the necessary strength to support the dead rim weight and the weight of the material which forms the remainder of the disk. The strength of these superalloys peaks at a temperature well below the temperature of the working medium gases and decreases as the temperature of the alloy is raised above the peak.
Rotor blades designed for use with high temperature working medium gases have a platform which blocks the transfer of heat from the hot, working medium gases to the disk. Although the platform performs this useful function, the weight of the platform increases the dead rim weight of the disk causing an increase in the level of stresses or an increase in the size of the disk for the same fatigue life.
Accordingly, scientists and engineers are working to develop a rotor blade which orients the root of the blade more parallel to the axis of rotation of the disk than was permitted by the prior art designs to reduce stress concentrations in the disk, which has an airfoil which satisfies stress and aerodynamic demands placed on the rotor blade and which has a platform for shielding the disk and the root of the rotor blade from the hot working medium gases without compromising the structural integrity of the overall design.